BTP estimates the FG based on the OG and the attenuation value only. It does not take into account the source of the gravity points. It is up to you to enter a different attenuation value.

You can estimate this new value by taking into account that the sugar is completely fermentable. Temporarily remove the sugar from the recipe (uncheck it) and, if the attenuation is typical for you, the new FG should be a closer approximation. Add the sugar back, then change the attenuation to get that FG.